The invention relates to a method of imparting high strength, high ductility and high toughness to an alloy, and the resulting article. In preferred embodiments, the method includes a plurality of working steps that effect a predetermined reduction in the cross-sectional area of a liquid phase sintered tungsten heavy alloy workpiece.
It is known to plastically work refractory metal alloys to improve the strength thereof. Typically, these materials exhibit increased strength and increased hardness in proportion with increased reduction in cross-sectional area of the workpiece being worked.
Previously, certain refractory metal alloys, such as liquid-phase-sintered tungsten heavy alloys were mechanically worked in the range of 7% to 25% reduction in cross-sectional area in order to produce a high strength material. Working the material beyond about 25% using conventional techniques has been found to produce defects at the matrix/tungsten interface. Also, working the alloy in this manner results in a significant reduction in ductility and/or fracture toughness.
Often it is desirable to produce an alloy having a combination of properties, such as high ductility, high fracture toughness, as well as high strength. Previously, such a combination of properties could only be obtained by working the material to a total reduction in area on the order of about 95%, or greater. Applying this much work to the alloy workpiece is costly, time consuming, and makes it difficult, if not impossible, to produce certain larger, more complex shapes.
U.S. Pat. No. 4,990,195 to Spencer et al. discloses a process for producing solid-state sintered only tungsten heavy alloy articles that includes forming a bar from the tungsten heavy alloy material and working the bar to achieve a total reduction in area of at least 80%.
U.S. Pat. No. 4,762,559 to Penrice et al. discloses a high density tungsten-based alloy with a matrix of nickel-iron-cobalt and method for making the same which includes swaging a sintered compacted body to effect a total reduction in area of 5% to 40%, and typically 20% to 25%.
U.S. Pat. No. 5,523,048 to Stinson et al. discloses a method for producing high density refractory metal warhead liners that includes forming a near net-shaped blank from pure or solid-solution-alloy molybdenum or tungsten powder, and optionally subjecting this workpiece to a singular forging step. The amount of reduction in cross-sectional area effected by this forging step is not disclosed.
The method of the present invention produces an article possessing a beneficial combination of properties including high ductility, high fracture toughness, and high strength.
These and other beneficial results can be obtained by subjecting a refractory metal alloy to a process including: (i) subjecting the workpiece to a first cold or warm working step including at least one pass that reduces the initial cross-sectional area of said material, (ii) annealing the workpiece subsequent to the at least one pass, and (iii) subjecting the alloy to a final working step comprising at least one pass conducted at a temperature between ambient and 300xc2x0 C., the final working step further reducing the cross-sectional area of the workpiece such that the overall total reduction in the initial cross-sectional area of the workpiece effected by all working steps is approximately 40%-75%.
The invention also encompasses the resulting article which possesses a tensile yield strength of approximately 170-200 Ksi, a tensile elongation of approximately 12%-17%, and a Charpy 10 mm Smooth Bar impact toughness of approximately 100 ft.-lb. to 240 ft.-lb.